In the automobile industry, replacement of metallic materials with plastics in various automobile parts has been promoted recently for the purpose of weight reduction and energy saving. Polyolefin resins are generally used for this purpose from the standpoint of cost and environmental conservation as well as product performance such as strength, weather resistance and chemical resistance.
Of the polyolefin resins, polyethylene is particularly suited to blow molding. Polyethylene having a relatively broad molecular weight distribution, high melt tension, and uniform stretchability is generally employed. In particular, large-sized blown polyethylene containers, such as fuel tanks or drums, have been attracting attention.
Fuel tanks made of high-molecular-weight and high-density polyethylene have large freedom of shape as compared with conventional steel-made fuel tanks and are mounted on such types of automobiles as 4 wheel drive cars or 4 wheel steering cars.
Ethylene polymers suitable for such use proposed to date include ethylene copolymers with excellent molding properties and environmental stress cracking resistance (hereinafter abbreviated as ESCR) as disclosed in Unexamined Published Japanese Patent Application No. 53811/90.
However, when high-density polyethylene is molded into fuel tanks of complicated shape, the wall of the resulting tank becomes thinner and weaker at bends so that the wall thickness of the whole tank must be increased, which leads to disadvantages for economy and weight reduction.
Where ethylene is copolymerized with an .alpha.-olefin aiming at improvements in mechanical properties, such as impact resistance and ESCR, the resulting copolymer, which generally has a relatively low density, has reduced stiffness. With the purpose of weight and thickness a reduction in mind, reduction in stiffness would cause deflection or deformation of fuel tanks on use or when piled up.